1. Field of the Invention
The present invention generally relates to a thin film magnetic memory device. More particularly, the present invention relates to a random access memory (RAM) including memory cells having a magnetic tunnel junction (MTJ).
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) device has attracted attention as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and also capable of random access to each thin film magnetic element.
In particular, recent announcement shows that the performance of the MRAM device is significantly improved by using thin film magnetic elements having a magnetic tunnel junction (MTJ) as memory cells. The MRAM device including memory cells having a magnetic tunnel junction is disclosed in technical documents such as xe2x80x9cA 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cellxe2x80x9d, ISSCC Digest of Technical Papers, TA7.2, February 2000, and xe2x80x9cNonvolatile RAM based on Magnetic Tunnel Junction Elementsxe2x80x9d, ISSCC Digest of Technical Papers, TA7.3, February 2000.
FIG. 66 is a schematic diagram showing the structure of a memory cell having a magnetic tunnel junction (hereinafter, also simply referred to as xe2x80x9cMTJ memory cellxe2x80x9d).
Referring to FIG. 66, the MTJ memory cell includes a tunnel magnetic resistive element TMR having its electric resistance value varying according to the storage data level, and an access transistor ATR. The access transistor ATR is formed from a field effect transistor, and is coupled between the tunnel magnetic resistive element TMR and ground voltage Vss.
For the MTJ memory cell are provided a write word line WWL for instructing data write operation, a read word line RWL for instructing data read operation, and a bit line BL serving as a data line for transmitting an electric signal corresponding to the storage data level in the data read and write operations.
FIG. 67 is a conceptual diagram illustrating the data read operation from the MTJ memory cell.
Referring to FIG. 67, the tunnel magnetic resistive element TMR has a magnetic layer FL having a fixed magnetic field of a fixed direction hereinafter, also simply referred to as xe2x80x9cfixed magnetic layer FLxe2x80x9d), and a magnetic layer VL having a free magnetic field (hereinafter, also simply referred to as xe2x80x9cfree magnetic layer VLxe2x80x9d). A tunnel barrier TB formed from an insulator film is provided between the fixed magnetic layer FL and free magnetic layer VL. According to the storage data level, either a magnetic field of the same direction as that of the fixed magnetic layer FL or a magnetic field of the direction different from that of the fixed magnetic layer FL has been written to the free magnetic layer VL in a non-volatile manner.
In the data read operation, the access transistor ATR is turned ON in response to activation of the read word line RWL. As a result, a sense current Is flows through a current path formed from the bit line BL, tunnel magnetic resistive element TMR, access transistor ATR and ground voltage Vss. The sense current Is is supplied as a constant current from a not-shown control circuit.
The electric resistance value of the tunnel magnetic resistive element TMR varies according to the relative relation of the magnetic field direction between the fixed magnetic layer FL and free magnetic layer VL. More specifically, when the fixed magnetic layer FL and free magnetic layer VL have the same magnetic field direction, the tunnel magnetic resistive element TMR has a smaller electric resistance value as compared to the case where both magnetic layers have different magnetic field directions. The electric resistance values of the tunnel magnetic resistive element corresponding to the storage data xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d are herein represented by Rh and Rl, respectively (where Rh greater than Rl).
Thus, the electric resistance value of the tunnel magnetic resistive element TMR varies according to an externally applied magnetic field. Accordingly, data storage can be conducted based on the variation characteristics of the electric resistance value of the tunnel magnetic resistive element TMR.
A voltage change produced at the tunnel magnetic resistive element TMR by the sense current Is varies depending on the magnetic field direction stored in the free magnetic layer VL. Therefore, by starting supply of the sense current Is with the bit line BL precharged to a high voltage, the storage data level in the MTJ memory cell can be read by monitoring a change in voltage level on the bit line BL.
FIG. 68 is a conceptual diagram illustrating the data write operation to the MTJ memory cell.
Referring to FIG. 68, in the data write operation, the read word line RWL is inactivated, so that the access transistor ATR is turned OFF. In this state, a data write current for writing a magnetic field to the free magnetic layer VL is applied to the write word line WWL and bit line BL. The magnetic field direction of the free magnetic layer VL is determined by combination of the respective directions of the data write currents flowing through the write word line WWL and bit line BL.
FIG. 69 is a conceptual diagram illustrating the relation between the direction of the data write current and the direction of the magnetic field in the data write operation.
Referring to FIG. 69, a magnetic field Hx of the abscissa indicates the direction of a magnetic field H(BL) produced by the data write current flowing through the bit line BL. A magnetic field Hy of the ordinate indicates the direction of a magnetic field H(WWL) produced by the data write current flowing through the write word line WWL.
The magnetic field direction stored in the free magnetic layer VL is updated only when the sum of the magnetic fields H(BL) and H(WWL) reaches the region outside the asteroid characteristic line shown in the figure. In other words, the magnetic field direction stored in the free magnetic layer VL is not updated when a magnetic field corresponding to the region inside the asteroid characteristic line is applied.
Accordingly, in order to update the storage data of the tunnel magnetic resistive element TMR by the data write operation, a current must be applied to both the write word line WWL and bit line BL. Once the magnetic field direction, i.e., the storage data, is stored in the tunnel magnetic resistive element TMR, it is retained therein in a non-volatile manner until another data write operation is conducted.
The sense current Is flows through the bit line BL in the data read operation. However, the sense current Is is generally set to a value that is about one to two orders smaller than the data write current. Therefore, it is less likely that the storage data in the MTJ memory cell is erroneously rewritten by the sense current Is during the data read operation.
The magnetization characteristics of the magnetic layers of each MTJ memory cell significantly affect the memory cell characteristics. In particular, when a change in magnetization direction for data storage becomes less likely to occur in the tunnel magnetic resistive element TMR due to end effects of the magnetic element or the like, the magnetic field required for the data write operation is increased, causing increase in power consumption and magnetic noise due to the increased data write current. Moreover, a variation in electric resistance value depending on the storage data level is reduced, causing reduction in signal margin in the data read operation.
In the MRAM device using the tunnel magnetic resistive element, reduction in memory cell size is difficult for the structural reason. In particular, it is difficult to realize the folded-bit-line structure that is effective in improving a signal margin in the data read operation and is generally applied to a dynamic random access memory (DRAM) or the like.
Moreover, in the folded-bit-line structure, complementary bit lines forming a bit line pair are respectively coupled to a memory cell to be read and a read reference voltage. By amplifying the voltage difference between the complementary bit lines, the data read operation is conducted with a large signal margin. Accordingly, the read reference voltage must be set in view of the electric resistance values Rh and Rl of the tunnel magnetic resistive element. However, it is difficult to accurately set the read reference voltage while allowing manufacturing variation.
It is an object of the present invention to provide a thin film magnetic memory device including memory cells using a tunnel magnetic resistive element having uniform magnetization characteristics.
It is another object of the present invention to provide a thin film magnetic memory device capable of ensuring a large signal margin in the data read operation while allowing manufacturing variation.
It is still another object of the present invention to provide a thin film magnetic memory device having a memory cell arrangement suitable for improved integration, in particular, a memory cell arrangement suitable for a folded-bit-line structure.
In summary, according to the present invention, a thin film magnetic memory device formed on a semiconductor substrate includes a plurality of memory cells for storing data. Each memory cell includes an access element rendered conductive for forming a path of a data read current, and a magnetic storage portion coupled in series with the access element and having an electric resistance varying according to storage data. The thin film magnetic memory device further comprises a first magnetic layer formed on the semiconductor substrate and having a fixed magnetization direction, a second magnetic layer formed on the semiconductor substrate and magnetized in a direction according to an externally applied magnetic field, and an insulating film formed between the first and second magnetic layers. The magnetic storage portion is formed using a prescribed partial region in a planar direction of the second magnetic layer.
Accordingly, a primary advantage of the present invention is that the magnetic storage portion in each memory cell can be formed so as to have uniform magnetization characteristics. This assures a signal margin of the data read operation as well as reduces a data write current required for the data write operation, allowing for suppression in current consumption and magnetic noise.
According to another aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a dummy memory cell, a first data line, a second data line, and a data read circuit. An electric resistance value of each memory cell varies according to a storage data level. The dummy memory cell produces a read reference voltage. The dummy cell includes a plurality of cell units each having a same structure as that of the memory cell. The plurality of cell units retain storage data of different levels at least on a one-by-one basis. The first data line is connected to a selected one of the plurality of memory cells in data read operation. The second data line is connected to the dummy memory cell. The data read circuit senses a voltage difference between the first and second data lines.
Accordingly, the read reference voltage can be produced based on the data stored in the cell units having the same structure as that of the memory cell. As a result, the data read operation can be conducted with a large signal margin by setting the read reference voltage to an appropriate level while allowing manufacturing variation.
According to still another aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read word lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first and second contacts are repeatedly arranged in a same manner in every memory cell row. The memory cells are shifted by xc2xd pitch between adjacent memory cell columns. The write word lines are each formed in a layer located above the bit lines.
Thus, the memory cells corresponding to each read word line are connected to every other bit line. Therefore, the memory cell arrangement suitable for the data read operation based on the folded-bit-line structure can be realized without increasing the cell size. Moreover, the distance between the magnetic storage portions can be increased as compared to the case where the memory cells are not shifted. This suppresses magnetic-field interference between the memory cells, whereby an operation margin can be ensured. The memory cell pitch in the row direction can be easily ensured, allowing for improved integration of the memory array.
According to yet another aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read world lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first and second contacts are inverted in position between adjacent memory cell rows. The memory cells are shifted by prescribed pitch between adjacent memory cell columns. The write word lines are each formed in a layer located above the bit lines.
Thus, the distance between the magnetic storage portions can be increased as compared to the case where the memory cells are not shifted. This suppresses magnetic-field interference between the memory cells, whereby an operation margin can be ensured. The memory cell pitch in the row direction can be easily ensured, allowing for improved integration of the memory array.
According to a further aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read world lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first and second contacts are repeatedly arranged in a same manner in every memory cell row. The first and second contacts are inverted in position between adjacent memory cell columns. The write word lines are each formed in a layer located above the bit lines.
Thus, the distance between the magnetic storage portions can be increased. This suppresses magnetic-field interference between the memory cells, whereby an operation margin can be ensured. The memory cell pitch in the row direction can be easily ensured, allowing for improved integration.
According to a still further aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read world lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first and second contacts are repeatedly arranged in a same manner in every memory cell row. The first and second contacts are inverted in position between adjacent memory cell columns. The memory cells are shifted by xc2xd pitch between adjacent memory cell columns.
Thus, the memory cells corresponding to each read word line are connected to every other bit line. Therefore, the memory cell arrangement suitable for the data read operation based on the folded-bit-line structure can be realized without increasing the cell size.
According to a yet further aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read world lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first and second contacts are inverted in position between adjacent memory cell rows. The first and second contacts are inverted in position between adjacent memory cell columns. The write word lines are each formed in a layer located above the bit lines.
Thus, the memory cell arrangement suitable for the data write operation based on the folded-bit-line structure can be realized without increasing the cell size. Moreover, the memory cell pitch in the row direction can be easily ensured, allowing for improved integration of the memory array.
According to a yet further aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read world lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first and second contacts are inverted in position between adjacent memory cell rows. The first and second contacts are inverted in position between adjacent memory cell columns. The memory cells are shifted by xc2xc pitch between adjacent memory cell columns. The write word lines are each formed in a layer located above the bit lines.
Thus, the memory cells corresponding to each read word line are connected to every other bit line. Therefore, the memory cell arrangement suitable for the data read operation based on the folded-bit-line structure can be realized without increasing the cell size.
According to a yet further aspect of the invention, a thin film magnetic memory device includes a plurality of memory cells, a plurality of read world lines, a plurality of write word lines, and a plurality of bit lines. The plurality of memory cells are arranged in rows and columns. The plurality of read word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data read operation. The plurality of write word lines are provided respectively corresponding to the memory cell rows, for conducting row selection in data write operation. The plurality of bit lines are provided respectively corresponding to the memory cell columns, for passing therethrough a data write current and a data read current in the data write and read operations, respectively. Each of the plurality of memory cells includes a magnetic storage portion having an electric resistance varying according to storage data, and an access transistor coupled in series with the magnetic storage portion between a corresponding bit line and a first voltage. The access transistor includes a gate coupled to a corresponding read word line, a first contact for coupling a source region to the first voltage, and a second contact provided adjacent to the first contact in the column direction, for coupling a drain region to the magnetic storage portion. The first contact is shared by corresponding two memory cells located adjacent to each other in the column direction and forming a single arrangement unit. The write word lines are each formed in a layer located above the bit lines.
Thus, the memory cells can be arranged with a reduced number of contacts of the access transistors.
According to a yet further aspect of the present invention, a thin film magnetic memory device includes a plurality of memory cells for retaining storage data. Each of the memory cells includes an access gate selectively turned ON in data read operation, and a magnetic storage portion connected in series with the access gate, and having either a first or second electric resistance depending on the storage data. The magnetic storage portion includes a first magnetic layer having a fixed magnetization direction, a second magnetic layer that is magnetized either in a same direction as, or in a direction opposite to, that of the first magnetic layer depending on the storage data to be written, and a first insulating film formed between the first and second magnetic layers. The thin film magnetic memory device further includes: a data line that is electrically coupled to the magnetic storage portion of a selected memory cell through a turned-ON access gate of the selected memory cell in data read operation, the selected memory cell being a memory cell selected from the plurality of memory cells for the data read operation; a reference data line for transmitting in the data read operation a read reference voltage for comparison with a voltage on the data line; and a plurality of dummy memory cells for producing the read reference voltage, each of the dummy memory cells being provided for every fixed set of the memory cells. Each of the dummy memory cells includes a dummy magnetic storage portion, and a dummy access gate selectively turned ON in the data read operation, for electrically coupling the dummy magnetic storage portion to the reference data line. The dummy magnetic storage portion includes a third magnetic layer that is magnetized in a fixed direction, a fourth magnetic layer that is magnetized in a direction that crosses the magnetization direction of the third magnetic layer, and a second insulating film formed between the third and fourth magnetic layers.
Such a thin film magnetic memory device is capable of setting an electric resistance of the dummy magnetic storage portion having the same structure as that of the magnetic storage portion in the memory cell to an intermediate value of two electric resistances of the memory cell each corresponding to the storage data. This allows a dummy memory cell for producing a read reference voltage to be fabricated without complicating the manufacturing process.
According to a yet further aspect of the present invention, a thin film magnetic memory device includes a plurality of memory cells for retaining storage data. Each of the memory cells includes an access gate selectively turned ON in data read operation, and a magnetic storage portion connected in series with the access gate, and having either a first electric resistance or a second electric resistance higher than the first electric resistance depending on the storage data. The magnetic storage portion includes a first magnetic layer having a fixed magnetization direction, a second magnetic layer that is magnetized in a same direction as, or in a direction opposite to, that of the first magnetic layer depending on the storage data to be written, and a first insulating film formed between the first and second magnetic layers. The thin film magnetic memory device further includes: a data line that is electrically coupled to the magnetic storage portion of a selected memory cell through a turned-ON access gate of the selected memory cell in data read operation, the selected memory cell being a memory cell selected from the plurality of memory cells for the data read operation; a reference data line for transmitting in the data read operation a read reference voltage for comparison with a voltage on the data line; and a plurality of dummy memory cells for producing the read reference voltage, each of the dummy memory cells being provided for every fixed set of the memory cells. Each of the dummy memory cells includes a dummy access gate selectively turned ON in the data read operation, and a plurality of dummy magnetic storage portions that are electrically coupled to the reference data line in response to turning-ON of the dummy access gate. Each of the dummy magnetic storage portions includes a third magnetic layer that is magnetized in a fixed direction, a fourth magnetic layer that is magnetized either in a same direction as, or in a direction opposite to, that of the third magnetic layer, and a second insulating film formed between the third and fourth magnetic layers. Each of the dummy magnetic storage portions is connected in series with at least one of the remainder.
Such a thin film magnetic memory device is capable of producing a read reference voltage by a dummy memory cell that includes a dummy magnetic storage portion having the same structure and magnetized in the same manner as that of the magnetic storage portion of the memory cell. This enables fabrication of the dummy memory cell without complicating the manufacturing process. Moreover, a reduced voltage can be applied to a tunnel barrier (second insulating film) in each dummy memory cell, allowing for improved reliability of the dummy memory cell that is selected frequently.
According to a yet further aspect of the present invention, a thin film magnetic memory device includes: a plurality of magnetic memory cells for retaining storage data written by an applied magnetic field; and a dummy memory cell for generating a read reference voltage in data read operation. Each of the magnetic memory cells and the dummy memory cell include a magnetic storage portion having either a first electric resistance value or a second electric resistance value that is higher than the first electric resistance value depending on a level of the storage data, and an access gate connected in series with the magnetic storage portion, and selectively turned ON. The thin film magnetic memory device further includes: a first data line that is electrically coupled to a magnetic memory cell selected from the plurality of magnetic memory cells in data read operation so that a data read current is supplied to the first data line; a second data line that is electrically coupled to the dummy memory cell in data read operation so that a data read current equal to that of the first data line is supplied to the second data line; a data read circuit for producing read data based on respective voltages on the first and second data lines; and a resistance adding circuit for adding a third electric resistance in series with the first data line, the third electric resistance being smaller than a difference between the first and second electric resistance values. The magnetic storage portion in the dummy memory cell stores a data level corresponding to the second electric resistance value.
Such a thin film magnetic memory device enables the memory cell and the dummy memory cell to have the same structure, allowing a data read margin to be assured according to manufacturing variation.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.